1. Field of the invention
This invention relates to self-adhesive carbonaceous grains suitable for the manufacture of high-density and high-strength carbon artifacts. The invention also relates to a process for producing high-density and high-strength carbon artifacts showing a fine mosaic texture of optical anisotropy from self-adhesive carbonaceous grains. More particularly, the invention relates to a process for producing high-density and high-strength carbon artifacts characterized by random arrangement of optically anisotropic units at the submicron level.
2. Prior Art
Many approaches have heretofore been known in the manufacture of high-density carbon artifacts. They are generally produced by mixing fillers such as pulverized coke, natural graphite and carbon black with binders such as coal-tar pitch, molding the mixture and baking the mold. The carbonization yield of the conventional binder is very low and the density of the mold achieved by single carbonization is accordingly very low; therefore, the steps of impregnation and carbonization must be repeated until the desired properties are attained. As a further problem, the major light-weight components of the binder will evaporate during the carbonization to form pores that remain in the mold; such pores will introduce inhomogeneity in the mold. Additionally, structure disruption may be caused by the bloating of the binder. To avoid these adverse effects, the carbonization step must be performed by heating at an extremely slow rate but then this increases the time schedule of carbon artifact production to at least 3-4 weeks. The carbonized mold may be graphitized by further heating at 2500.degree.-3000.degree. C., depending on the use of the final product. Besides, a period of 2-3 weeks is required to perform this step of graphitization. In total, a period as long as 2-3 months is taken to manufacture graphitic artifacts prepared from fillers (e.g. coke) and binders (e.g. coal-tar pitch) through the above-described complex route.
As the demand for improvements in the performance of specialty carbon artifacts and composites is growing year by year, controlling the structure and texture of carbon is extremely important for the purpose of enhancing the performance of the final product since the physicochemical characteristics of the product significantly depend on the structure and texture of carbon. In the field of specialty carbon artifacts, many R&D efforts have been made to control the shape and size of starting grains so that the carbon artifacts will exhibit fine mosaic texture, thereby achieving not only higher density and strength but also physical isotropy.
A method is known to attain physical isotropy of the artifact by using mesocarbon microbeads as a starting material. In this method, the optically anisotropic small spheres that form in the process of heat treatment of coal-tar, petroleum-derived heavy oils, etc. at temperatures of 350.degree.-500 .degree. C. are solvent-extracted from the pitch matrix, dried, molded under pressure and baked. However, as pointed out in Unexamined Published Japanese Patent Application (KOKAI) No. Hei 1-239058, the size of optical unit of the carbon artifact produced by that method is not smaller than the particle size of mesophase spheres (10-20 .mu.m) and it is impossible to decrease the size of optical unit. A further problem with the method is that an extremely large quantity of extraction solvent is required in the step of separating the spheres. In addition, it is difficult to remove the remaining solvent completely from the recovered spherical grains and this can be a cause of cracking or expansion of the mold in the subsequent carbonization step. Further, in addition to the extremely low yield of the spheres obtained by solvent extraction, it is difficult to control their properties. In other words, this method is not feasible since it is not easy to prepare mesocarbon microbeads of acceptable quality and price on an industrial scale.
Another method is concerned with the grains of a pulverized bulk mesophase of specified properties as a starting material (see Examined Japanese Patent Publication (KOKOKU) No. Hei 1-58124). However, the carbon artifacts produced from the pulverized bulk mesophase show low bulk density, and no satisfactory performance has been achieved. As a further problem, the bulk mesophase, which is obtained by coalescing and agglomerating mesocarbon microbeads, has to be separated from the pitch matrix and many complex processing steps are required to obtain a bulk mesophase of specified properties. It has also been pointed out in Unexamined Published Japanese Patent Application (KOKAI) No. Hei 1-239058, supra, that the size of optically anisotropic unit derived from pulverized bulk mesophase in the mold cannot become smaller than that achieved by pulverization.
Many cases of the attempt to use particular grains that are preliminarily modified to exhibit a fine mosaic texture have also been reported. For example, Examined Japanese Patent Publication (KOKOKU) No. Sho 58-58284 teaches a method in which semi-coke with a mosaic texture composed of extremely fine optical units (.ltoreq.1 .mu.m) is used as a molding feed. However, this method involves a complex process including solvent-extraction of the feed coal in the presence of hydrogen gas, separation of the extract, followed by heat treatment. Examined Japanese Patent Publication (KOKOKU) No. Hei 3-6448 teaches a method of adding carbon black to pitch, and Unexamined Published Japanese Patent Application (KOKAI) No. Hei 1-239058, supra, teaches a process for producing an isotropic graphite artifact having a homogeneous mosaic texture by the steps of incorporating a resin in pitch, pulverizing the mixture, molding the grains in the absence of a binder, and baking the mold. However, both methods have the disadvantage of involving a complex procedure comprising mixing, kneading and re-pulverization. In addition, the carbon artifacts produced by these methods exhibit low bulk density, and no satisfactory performance has been attained.
Extensive studies have also been conducted in the area of carbon composites. For instance, International Symposium on Carbon, Toyohashi, Extended Abstract, p. 196, 1982 reported the superiority of matrix carbon showing a fine mosaic texture as regards the development of excellent thermal shock resistance and high mechanical strength; CARBON, vol. 28, 1990 reports a pitch/phenolic resin system (p. 559) and a pitch/carbon black system (p. 143), together with their interaction and carbonization properties.
As will be understood from the foregoing discussion, the manufacture of high-density carbon artifacts involves extremely complex and time-consuming processes and, hence, the products of the conventional methods are expensive enough to substantially limit the scope of their industrial applicability. Under the circumstances, one major goal in carbon industry in connection with the manufacture of high-density carbon artifacts is substantial simplification of the production process while shortening the time required for their production. As for the control of carbon texture which is the key factor to the enhancement of product performance and structural homogenization, the conventional processes have had various drawbacks as described hereinabove.
The present inventors previously found that self-adhesive carbonaceous grains suitable for the manufacture of high-density carbon artifacts could be prepared from a specified mesophase pitch and that this could be used as a means to solve the aforementioned problems of the prior art. The inventors formulated their finding in a patent application, which was filed in the United States of America as U.S. Ser. No. 08/063,421. This prior application teaches that self-adhesive carbonaceous grains defined in terms of H/C and O/C values have outstanding performance. However, the application makes no reference to the quinoline-soluble and pyridine-insoluble component or the quinoline-insoluble component of the carbonaceous grains, nor does it teach that carbon artifacts derived from said carbonaceous grains are characterized by random orientation of optically anisotropic units at the submicron level.